1. Field of the Invention
This invention is concerned with devices, such as mirrors and windows, having controllable reflectivity.
2. Description of the Related Art
Sunlight transmitted through windows in buildings and transportation vehicles can generate heat (via the greenhouse effect) that creates an uncomfortable environment and increases air conditioning requirements and costs. Current approaches to providing xe2x80x9csmart windowsxe2x80x9d with adjustable transmission for use in various sunlight conditions involve the use of light absorbing materials. These approaches are only partially effective, since the window itself is heated and because these devices, such as electrochromic devices, are relatively expensive and exhibit limited durability and cycle life. Certain liquid crystal-based window systems switch between transmissive and opaque/scattering states, but these systems require substantial voltages to maintain the transparent state. There is an important need for an inexpensive, durable low voltage smart window with variable reflectivity. Reflecting the light, rather than absorbing it, is the most efficient means for avoiding inside heating. Devices for effectively controlling transmission of light are also needed for a variety of other applications, e.g., energy efficient dimmers for displays.
Bright light from headlamps on following vehicles reflected in automobile rear and side view mirrors is annoying to drivers and creates a safety hazard by impairing driver vision. Currently available automatically dimming mirrors rely on electrochromic reactions to produce electrolyte species that absorb light that would otherwise be reflected from a static mirror. Such devices do not provide close control over the amount of reflected light, and are expensive to fabricate since a very constant inter-electrode spacing is required to provide uniform dimming. Image sharpness is also reduced for electrochromic mirror devices since the reflected light must pass through the electrolyte (twice). There is an important need for an inexpensive adjustable mirror device that provides close control of reflected light with minimal image distortion.
In prior art attempts to exploit reversible electrodeposition of a metal for light modulation, the deposits obtained on transparent substrates presented a rough and black, gray, or sometimes colored appearance (typical of finely-divided metals) and exhibited poor reflectivity and high light absorbance, especially when thick. Such deposits have been investigated for display applications involving reflectance from the background, with white pigments often being added to improve contrast. Warszawski (U.S. Pat. No. 5,056,899), which is concerned with displays, teaches that reversible metal electrodeposition is most appropriate for display applications, since significant disadvantages for transmission devices were given (e.g., the possibility of metal deposition at the counter electrode). In general, the prior art literature teaches that an auxiliary counter electrode reaction is required for transmission-type devices to avoid metal electrodeposition at the counter electrode as metal electrodissolution occurs at the working electrode, which would produce no net change in transmission. Such teachings imply that the application of reversible metal deposition to smart windows must involve light absorption by the finely divided electrodeposited metal, which would result in heating of the device itself and thus the space inside. The low reflectance of this type of deposit would not be appropriate for adjustable mirror applications.
Electrolytes described in the prior art literature contain auxiliary redox species (e.g., bromide, iodide, or chloride) that are oxidized (e.g., to bromine, iodine, or chlorine) at the counter electrode during metal deposition under the high drive voltages used. This introduces chemistry-related instabilities during long term operation and leads to deposit self erasure on open circuit via chemical dissolution of the metal deposit, e.g., 2Ag0+Br2--- greater than 2AgBr. In most cases, this auxiliary redox process hinders metal deposition at the counter electrode during erasure, introducing a threshold voltage that is desirable for display applications. This auxiliary redox process may represent a significant side reaction even when metal electrodeposition/dissolution occurs at the counter electrode and a threshold voltage is not observed. See, e.g., Warszawski, columns 3-4 (when copper or nickel were present in the counter electrode paste) and Duchene et al., Electrolytic Display, IEEE Transactions on Electron Devices, Volume ED-26, Number 8, Pages 1243-1245 (August 1979); French Patent No. 2,504,290 (Oct. 22, 1982). High switching voltages of at least 1 V were used for all the electrodeposition devices which have been found in the patent and literature prior art.
A paper by Ziegler et al. (Electrochem. Soc. Proc. Vol. 93-26, p. 353, 1993) describes an investigation for display applications of the reversible electrodeposition of bismuth in aqueous solutions containing a large molar concentration ratio of halide anions to the trivalent bismuth ion. Halide anion oxidation served as the counter electrode reaction with the 1.5 V write voltage used. The deposits obtained were dark in color and were shown to decrease the reflectance of the ITO surface. Subsequent reports by these authors (Electrochem. Soc. Proc. Vol. 94-31 (1994), p. 23; Solar Energy Mater. Solar Cells 39 (1995), p. 317) indicated that addition of copper ions to the electrolyte was necessary to attain complete deposit erasure. These authors also utilized a counter electrode reaction other than metal electrodeposition/dissolution, and also never obtained a mirror deposit. Thus, Ziegler et al. provide no teachings relevant to the effect of electrolyte composition on the deposition/dissolution rate and quality of mirror electrodeposits.
Warszawski teaches that the use of a grid counter electrode would give a less uniform deposit since deposition on the transparent working electrode is highly localized in the vicinity of the counter electrode grid lines (a consequence of the very thin film of gel electrolyte used). Warszawski also teaches the use of an aqueous gel electrolyte to minimize sensitivity to atmospheric contaminants and to avoid the necessity of having a leak tight seal. Such electrolytes, however, have much more limited temperature and voltage operating ranges compared with organic-based electrolytes with high boiling solvents.
One effort to improve the deposit quality of the electrolytic solution used in a reversible electrodeposition process, described in U.S. Pat. No. 5,764,401 to Udaka et al., requires the addition of organic additives to the solution. Unfortunately, such additives are typically destroyed during the electrodeposition process, greatly limiting cycle life. Furthermore, this approach fails to produce highly-reflectivemirror-like deposits that are required for adjustable mirror applications and provide the superior heat rejection needed for smart windows.
U.S. Pat. No. 5,880,872 to Udaka teaches that the xe2x80x9cworkingxe2x80x9d electrode of a reversible electrodeposition structure is degraded, and its working life thereby shortened, by the high voltage required to dissolve the metal film deposited upon it. Udaka states that this consequence can be avoided by adding an alkali metal halide to the device""s electrolytic solution, preferably in an amount which provides an alkali metal halide to silver halide ratio of between 0.5 to 5. However, the described electrolytic formulation fails to provide the inherent stability, high quality deposits, good erasure and long cycle life needed for practical applications. Mirror deposits were never obtained.
Prior art literature teaches that the memory effect is temporary. This is a consequence of the occurrence of a counter electrode reaction other than metal electrodeposition/dissolution. The energetic oxidation products generated at the counter electrode can cause dissolution of the metal deposit on the working electrode either chemically on open circuit (slow) or electrochemically during short circuit (fast).
Nishikitani et al. (European Patent No. 0,618,477) teaches that the counter electrode in electrochromic devices for smart window applications can be a metal grid which is substantially transparent. Since no metal electrodeposition occurs in electrochromic devices, however, the grid in this case is used to provide a transparent electrode, not to maintain transparency by localizing metal deposition. In addition, to provide adequate electrical capacity for electrochromic devices, Nishikitani""s grid would need a very high surface area (at least 10 m2/g and preferably 50 to 5,000 m2/g) and a line width of 50 to 5,000 xcexcm; alternatively, a plurality of dots on a conducting substrate can be used, but the dots must contain fine particles having electrical capacitance of not less than 1 farad/g.
In describing his concept for a reversible electrodeposition light modulation device, Zaromb (S. Zaromb, J. Electrochem. Soc. 109, p. 903, 1962) recognized that the concentration of the electrodeposited metal should be as high as possible to permit fast electrodeposition without excessive metal ion depletion at the electrode, but sufficiently below the solubility limit to avoid precipitation during rapid electrodissolution of the metal deposit. For his devices, involving electrodeposition of dark silver deposits, this worker recommended an aqueous electrolyte containing AgI at a molar concentration in the range of 3 to 3.5 M (solubility limit 4 M), and addition of 7 M NaI to enhance the electrolyte conductivity.
Nonetheless, relatively low concentrations of electrodeposited metal ions have been used in subsequent work on reversible electrodeposition light modulation devices employing nonaqueous solvents. This is not surprising since it is commonly recognized by those skilled in the art that such ionic salts tend to be much less soluble in nonaqueous solvents, which typically have lower dielectric constants than water. In addition, high concentrations of ionic salts in nonaqueous solvents would be expected to result in significant ion pairing, which can lower the electrolyte conductivity and reduce the rate at which high quality deposits can be electrodeposited. U.S. Pat. No. 5,880,872 to Udaka claims use of excess halide added as Li, Na or K salts (from 0.5 to 5 times the concentration of the silver halide) to support dissolution of silver halide for optical devices, but describes dissolution of only 0.5 M AgBr in nonaqueous dimethylsulfoxide(DMSO) solvent. Likewise, U.S. Pat. Nos. 5,764,401 and 5,864,420 to Udaka et al. describe use of only 0.5 M AgI or AgBr in DMSO and dimethylformamide (DMF) solvents. For the Udaka devices, even a potential of 1 V provided only about 1 mA/cm2 of current. None of the Udaka electrolyte formulations yielded mirror deposits, good electrolyte stability, or devices with long cycle life.
The reversible electrochemical mirror (REM) device of this invention permits efficient and precise control over the reflection/transmission of visible light and other electromagnetic radiation. The mirror device includes a first electrode (or working electrode), on which a mirror deposit is reversibly electrodeposited and electrodissolved, and a second electrode (or counter electrode) at which occurs the reverse of the metal electrodeposition/dissolution process occurring at the first electrode. At least one of the electrodes (and its substrate) is substantially transparent to at least a portion of the spectrum of electromagnetic radiation. Typically, the transparent electrode is indium tin oxide (ITO) or fluorine doped tin oxide (FTO) deposited on a transparent glass (or plastic) pane which serves as the substrate. An electrolytic solution is disposed between the first and second electrodes such that ions of a metal which can electrodeposit on these electrodes are soluble in the electrolytic solution. The electrolytic solution described herein provides the inherent stability, high deposit quality, complete deposit erasure, long cycle life and fast switching needed for most practical applications.
When a negative electrical potential is applied to the first electrode relative to the second electrode, the applied potential causes deposited metal to be dissolved from the second electrode into the electrolytic solution and to be electrodeposited from the solution onto the first electrode as a mirror deposit, thereby affecting the reflectance of the REM device. An electrochemically stable surface modification layer deposited on the first electrode is usually required to facilitate substantially uniform nucleation of the electrodeposited metal in order to form a mirror deposit on the first electrode, such that the amount of deposited metal subsisting on the first electrode affects the reflectivity of the mirror for the radiation. The reflectivity of this mirror deposit can be selectively adjusted from near 0% to almost 100%, depending on the amount of metal deposited on the conducting film. Conversely, when the polarity is reversed and a positive electrical potential is applied to the first electrode relative to the second electrode, the applied potential causes deposited metal to be dissolved from the first electrode and electrodeposited from the solution onto the second electrode, thereby reducing the reflectivity of the mirror.
In various embodiments, at least one of the electrodes and its substrate, are substantially transparent to at least a portion of the spectrum of electromagnetic radiation. For an adjustable reflectivity device, such as an automatically dimmable mirror, either the first electrode and substrate are made transparent to enable adjustable reflectivity of light entering the device through the first electrode/substrate pane, or the second electrode and substrate are made transparent so that the radiation passes through the electrolyte to the mirror formed on the first electrode. The locally distributed electrode described in U.S. Pat. No. 5,903,382 to Tench et al., which is assigned to the same assignee as the present application, may be used to render the second electrode substantially transparent. For a device involving adjustable transmittance, such as a smart window, both electrodes are made substantially transparent to the radiation, as described in U.S. Pat. No. 5,923,456 to Tench et al., also assigned to the same assignee as the present application.
The first electrode and the surface modification layer may be disposed uniformly on a first substrate, or may be disposed in a pattern. The surface modification layer may be a thin layer (i.e., sufficiently thin to be nominally transparent) of an inert metal which is electrochemically more stable with respect to oxidation than the electrodeposited metal. An underlayer may be added between the first electrode and the surface modification layer to improve adhesion.
The electrolytic solution of this invention provides fast mirror switching with outstanding electrolyte stability, deposit quality, deposit erasure, current-voltage behavior, and cycle life performance. The solution contains an essentially nonaqueous solvent, electrodepositable mirror metal cations, e.g., Ag+ ions, at a molar concentration of more than 0.5 M, and approximately twice this molar concentration or more of halide and/or pseudohalide anions. High solubility for the mirror metal cations can only be attained in the presence of such an excess of halide/pseudohalide anions, which are added to the electrolyte as salts of electrochemically unreactive cations, e.g., Na+ or Li+ ions. A halide/pseudohalide to mirror metal cation concentration ratio of significantly more than 2:1 may be used to optimize the switching speed, mirror deposit characteristics and cycle performance. In some cases, appreciable amounts of water might be added to the electrolyte to suppress the freezing point of the electrolyte, for example, without significantly affecting the device performance. The electrolytic solution may include a gelling agent to form an essentially nonaqueous gel electrolyte, as well as dissolved or suspended materials to enhance light absorption (e.g., to provide a black background) or reflection (e.g., to provide a white background), impart color, and/or provide additional electrolyte stability.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.